1. Field of the Invention
The present invention relates to a scanning optical system, and more particularly to a scanning optical system for preventing ghosting caused by light reflected on surfaces of lenses in the system.
2. Description of the Related Art
A scanning optical system is essential to a laser printer, a laser scanner, a bar code reader and the like. An optical deflector such as a polygonal mirror, a hologram disk or the like is used for the system. A laser beam emitted from a semiconductor laser hits the light deflector and is scanned. The scanned light beam is further scanned on a scanning surface through the use of a scanning lens group. The scanning surface is a drum having a surface made of a photosensitive material in the case where the optical scanning system is applied to a laser printer.
Conventionally, glass has been used as the material for the scanning lenses of the scanning optical system, and an anti-reflection coating has been applied to the surface thereof. In recent years, however, the scanning lenses also have been made of synthetic resins to reduce production costs, and the anti-reflection coating has not been applied due to technical and economical reasons. Glass lenses with an anti-reflection coating have no reflection problems with a laser beam. But, plastic lenses without anti-reflection coating are one of the factors having a detrimental influence on the quality of pictures since ghosting caused by reflections between surfaces more frequently occurs with plastic lenses. The ghosting makes an image on the scanned surface unclear, for example, printed letters become unclear in laser beam printers. Moreover, the sensitivity of the photosensitive drum has been improved recently to indicate half tone images. Therefore, a reduction in the quality of images due to ghosting is not negligible.